A light guide is formed by a rod shaped transparent body and is used for a lighting device (line light source device). A lighting device including the light guide (line light source device) is provided with, for example, a light source such as an LED. Light emitted from the light source enters from an end face (light incident surface) of the light guide, is guided inside the light guide. The lighting device emits linear light from a light emission surface that is a side surface of the light guide. The lighting device (line light source device) including the light guide is advantageously used for lighting applications of image reading apparatuses such as a facsimile, a copying machine, a scanner, and a bill discriminator. Some image reading apparatuses include a lighting device (line light source device) inside a housing thereof.
Some conventional light guides include light guides having a recessed portion whose side is cutout (for example, refer to Patent Literature 1 and Patent Literature 2). In addition, some light guides include a light emission side surface from which light is emitted, the light emission side surface being a lens surface (refer to, for example, Patent Literature 3). Further, some light guides include a rod-shaped light guide whose vicinity of a light incident end face is thin (refer to, for example, Patent Literature 4 and Patent Literature 5).
When such light guides are used for a lighting device (line light source device), there are cases in which rotation of the light guide is prevented by holding an end face of the light guide using a light guide holder, and providing a recess in the light guide (for example, refer to Patent Literature 6). In many cases, light guide holders often have a function of suppressing leakage light that is light leaking from the vicinity of the end face of the light guide.
Leakage light from the vicinity of the end face of the light guide is preferably made uniform due to a large amount of the light being emitted without numerous repeated reflections inside the light guide. A light guide exists that is developed with the objective of making uniform the light emitted near the end face of the light guide by using a light diffusion concavo-convex portion formed on the side surface of the light guide (refer to, for example, Patent Literature 7).
Further, when the light guide is used for a lighting device (line light source device), there is a light guide holder including a peaked portion (refer to, for example, Patent Literature 8, and Patent Literature 9). Further, there are light guides including, besides the light guide holder, a connecting portion or a retaining portion that can block the light of the light guide and that covers a portion of the light guide or fixes the light guide (for example, Patent Literature 9, and Patent Literature 10).